This invention relates to electrostatic printing devices and more particularly to duplex printing.
Many methods for automatic duplex printing in xerographic processors are known. There is the two pass method employed in the Xerox 40000 and 9400(copyright) reproduction machines. That is, after the first side of copy sheets are imaged and fused, the sheets are collected in a duplex tray. After the last sheet in a set has been received in the duplex tray, the sheets are again passed through the xerographic processing stations. This time an image is transferred and fused onto the opposite side of each copy sheet having an image on the first side.
In the Xerox 9700(copyright) machine, the copy sheets also pass through the processing stations twice. However, they are not collected in a duplex tray. After the first image has been transferred and fused, the sheets pass through a stop and reverse mechanism (inverter). Then, the sheets join in an interleaving fashion the stream of copy sheets to receive an image on the opposite side.
There are some disadvantages with these systems, in particular for a given image throughput rate. For example, two passes through the fuser require more energy, and the fuser needs to operate at twice the speed. During the first pass through the fuser, the paper loses 50 percent of its moisture. This curls the paper and makes the second pass for duplexing difficult. Paper picks up oil on the first pass through the fuser, sometimes leading to image deletions on the second image and oil deposits on the photoreceptor. Jam rates during two-pass duplex operation are much greater than for simplex operation. In the first place, in a two-pass duplex system, the paper path is usually very long, and the paper has to negotiate all obstacles twice. Excessive paper curl is not only troublesome in the processor but also extremely difficult to handle in output stackers and finishing devices.
In other prior art systems such as in U.S. Pat. No. 4,095,979 means are shown for xe2x80x9cimmediatexe2x80x9d or single-pass duplex copying or forming first and second images sequentially on a photoreceptor. The first image is transferred from the photoreceptor to the first side of a copy sheet. Then, the sheet is stripped off the photoreceptor, inverted while the first image remains unfixed, and then the second image is transferred to the second side of the copy sheet. Both images are then fixed onto the copy sheet by a suitable fuser. This type of system can be described as a xe2x80x9csingle-passxe2x80x9d to the fuser.
Other single-pass duplex printing methods use intermediate image carriers (belt or drum). The first and second images are sequentially formed on a photoreceptor. The first image is transferred to an intermediate image carrier. The copy sheet is then passed between the photoreceptor and the intermediate image carrier, simultaneously receiving first and second images.
The duplex methods discussed above only utilize one photoreceptor. Other systems, e.g. U.S. Pat. Nos. 3,580,070 and 3,775,102 deal with xe2x80x9csingle-pass duplexxe2x80x9d methods employing two photoreceptors and two exposure systems. First, images are deposited on one photoreceptor and second images are deposited on the other photoreceptor. These systems are considered the ultimate duplex throughput systems since they produce twice the number of images of xe2x80x9ctwo-pass duplexxe2x80x9d systems at same process speed. These xe2x80x9csingle-pass duplexxe2x80x9d systems, however, generally require web paper feed in which the copy is spooled up on a roll or cut into individual sheets after fusing. This unfortunately, introduces additional components and complexity into the system.
U.S. Pat. No. 4,427,285 discloses a discrete copy sheet feed system rather than a web paper feed system. A two photoreceptor, xe2x80x9csingle-pass duplexxe2x80x9d apparatus is disclosed wherein two images are formed, one on each photoreceptor and then transferred to opposite sides of the image receiving sheet.
U.S. Pat. No. 4,714,939 discloses an electrographic reproduction apparatus, of the single-pass type, capable of producing simplex or duplex copies on a receiver sheet traveling in a continuous direction along a path. The reproduction apparatus comprises a first dielectric member movable along a first path, a portion of such first path being tangent to and on one side of the sheet travel path. Transferable images, corresponding to information to be reproduced, are sequentially formed on such first member. A second member is movable along a second path. One portion of such second path is tangent to the sheet travel path on the opposite side from the first path and another portion of the second path, spaced from such one portion, is located to position the second member in image transfer relation to the first dielectric member. An electrostatic field, reversible in its effective direction, is utilized to transfer a transferable image from the first dielectric member to the second member at the portion of the second path where the first and second members are in image transfer relation and transfer such image from the second member to one side of a receiver sheet traveling along its travel path at the location where the position of the first path is tangent to the sheet travel path and for producing a duplex copy, a second image is transferred from the first dielectric member to the opposite side of such receiver sheet at the location where the portion of the first path is tangent to the sheet travel path.
Another technique involving the use of only one photoconductor, utilizes an intermediate image transfer member to receive the first image formed on the photoconductor before transfer to a final support medium. The intermediate transfer member as disclosed in U.S. Pat. Nos. 3,671,118 and 3,697,170 is such a belt.
Some of the problems associated with the above techniques are in handling the unfixed image from first photoreceptor to the second photoreceptor can lead to image damage and complicate systems for handling an unfixed image through transport devices. Furthermore, to meet side one to side two registration requirements a re-registration device is needed.
There is provided a duplex/simplex printer for producing toner particle images on opposite sides of a substrate during a duplex mode or producing toner particle image on one side of the substrate during a simplex mode comprising: means for forming a first toner particle image on an imaging member with toner particles electrostatically charged to a first polarity; an image conditioner, being enable only during said duplex mode, for reversing the charge polarity of the particles of said first toner particle image on an imaging member; an intermediate member for receiving of said first toner particle image during said duplex mode; a bias transfer roll, positioned opposed from said imaging member with the intermediate member therebetween, for transferring said first image to said intermediate member by moving the intermediate member and the imaging members past said bias transfer roll being charged at said first polarity during said duplex mode, said duplex mode includes said forming means forming a second toner particle image on said imaging member with toner particles electrostatically charged to the first polarity and means for feeding a substrate between said intermediate member and said imaging and a controller reversing the charge polarity of said bias transfer roll to a second polarity thereby simultaneously transfer charged particles of said first and second images to opposite sides of said substrate during said duplex mode.